1. Field of the Invention
This invention relates to a dual purpose power limiting hydraulic system for carrying out two functions in the operation of an implement on a work vehicle, such as, a felling head on a tree harvesting machine.
2. Description of the Prior Art
U.S. Pat. No. 3,902,538 Muirhead shows a control system utilizing two different size pumps driven by a motor, for controlling a felling head which has a grabber or accumulator, and a cylinder for advancing the rotary cutter. The larger pump drives the cutter and has a valve which through a pressure sensing mechanism controls the speed of the cutter. A relief valve is preset to open at about 2200 PSI while another valve is opened only when it is necessary to vent the relief valve and thereby bring the cutter to a halt. The smaller capacity pump supplies pressurized fluid to the grabber arms and the cylinder.
U.S. Pat. No. 4,039,010 Tucek shows a hydraulic circuit for a tree harvesting implement having a cutter mounted on a movable frame with a hydraulic ram moving the frame and a hydraulic motor driving the cutter. The hydraulic circuit includes two separate pumps with one pump connected to one end of the fluid ram and the second pump connected to the hydraulic motor as well as the opposite end of the fluid ram with both conduits having pressure relief valves which are settable so that the frame is moved as a function of the resistance encountered by the cutter and/or the movable frame. As shown the second pump is driven by a motor which is driven by the first pump, but in the patent it is pointed out that both pumps can be driven directly from a single power source.
U.S. Pat. No. 4,083,291 Larsson shows a control system for a tree cutting implement which utilizes a chain saw. The motor of the saw is operated by a pump through a conduit. Another pump supplies pressurized fluid, through a flow divider, for operating feed cylinders which move the saw into the tree. When the pressure in the conduit to the motor rises above a predetermined amount such condition is signaled by electronic control apparatus which changes the position of the control valve to reduce the speed of flow in the conduit and thus slow down the rate of movement of the feed cylinders to the next lower preselected speed.
Some prior art tree processing implements are deficient in that only one pump supplies hydraulic fluid to the hydraulic cylinders. The size of the pump is chosen to be a compromise between rapid cylinder travel and limiting excess power consumption. High hydraulic flow at high pressure translates into heat, and thus inefficiency, when the limit of the pressure is determined by a relief valve.
Also in the prior art, pressure compensated variable displacement pumps have been used to accomplish rapid cylinder travel at low pressure while travel is slowed for high pressure conditions. These systems are deficient in that their cost is often higher than is considered acceptable. Furthermore, in such systems, the entire flow is at the highest pressure required by any function, which results in power waste during simultaneous hydraulic functions.
Also in the prior art, two stage pumps have been employed to operate tree processing implements, however, the deficiency of such a system is that the flow from one pump is limited by a relief valve when a predetermined pressure is reached, thus wasting energy. Furthermore, the pump that relieves at a relatively low pressure is incapable of serving any hydraulic function that requires a greater pressure.